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Harvard Medical School

Psoriatic Arthritis

What Is It?

Psoriatic arthritis is a chronic (long-lasting) disease in which a person with psoriasis develops the symptoms and signs of arthritis joint pain, stiffness and swelling. Psoriasis is a common, inherited skin condition that causes grayish-white scaling over a pink or dull-red skin rash.

Approximately 5% to 10% of the 3 million people who have psoriasis develop psoriatic arthritis. Psoriatic arthritis affects men and women equally and usually begins between ages 30 and 50. However, the disease can also occur in children. Most people have mild symptoms, but in some cases, the symptoms can be quite severe.

Types of Psoriatic Arthritis
There are five types of psoriatic arthritis. They are classified by their severity, whether both sides of the body are equally affected and which joints are involved.

• Asymmetric inflammatory arthritis – Often the knee, ankle, wrist or finger are involved, with a total of one to four inflamed joints. Usually, the arthritis does not affect both sides of the body equally (that is, the disease is not symmetric).

• Symmetric arthritis – Multiple joints are inflamed, often more than four, and the same joints on both sides of the body are affected. Fingernails often are ridged and pitted. This condition can mimic rheumatoid arthritis.

• Psoriatic spondylitis – One or both sacroiliac joints (the joints linking the spine and pelvis at the lower back), and sometimes other spine joints, are inflamed, causing morning stiffness in the back.

• Isolated finger involvement – This often involves only the last finger joint near the nail. One or more of these joints may be inflamed.

• Arthritis mutilans – This is the most severe and rarest form of psoriatic arthritis. In this form, the fingers shorten because of destruction of the joints and nearby bones.

Although each type of psoriatic arthritis is somewhat distinct, some people show a blending of symptoms or have more than one type.

Psoriasis can develop before or after the arthritis, but psoriasis develops first in about 75% of cases. A person may begin to get morning joint stiffness before the arthritis is recognized. People who have psoriasis that involves the nails, especially nail pitting, are much more likely to develop arthritis than those without this problem (50% versus 10%).

The cause of psoriatic arthritis is unknown. There is some evidence that infection or trauma can play a role in the development of the disease. For example, psoriatic arthritis seems to flare up in people whose immune systems are affected by human immunodeficiency virus (HIV) infection. Also, heredity seems to play a role. Up to 40% of people with psoriatic arthritis have a family history of skin or joint disease. Certain genes seem to be involved in certain types of psoriatic arthritis. For example, the gene HLA-B27 has been associated with psoriatic spondylitis.